A TFT active matrix substrate (hereinafter, referred to as a “TFT substrate”) using thin-film transistors (hereinafter, referred to as “TFTs”) as switching devices is used for optoelectronic devices, for example, displays using liquid crystals (liquid crystal displays: hereinafter, referred to as “LCDs”) and the like.
Liquid crystal displays (LCDs) are widely used for monitors of personal computers, portable information terminal devices, and the like, utilizing the advantages of low power consumption, compactness, and lightweight. In recent years, LCDs are widely used for television sets.
Generally, display modes of the LCD are roughly categorized into the TN (Twisted Nematic) mode and the lateral electric field mode represented by the in-plane switching mode and the FFS (Fringe Field Switching) mode. The liquid crystal display of the lateral electric field mode provides a feature of a wide viewing angle and a high contrast.
With respect to a liquid crystal display of the in-plane switching mode, display is performed by applying a lateral electric field to the liquid crystals held between opposing substrates, and pixel electrodes and a common electrode to which a lateral electric field is applied are provided on the same layer; thus, the liquid crystal molecules located right above the pixel electrodes are not sufficiently driven, whereby the transmittance is low.
On the other hand, in the FFS mode, because the common electrode and the pixel electrodes are disposed with an inter-layer insulating film therebetween, an oblique electric field (fringe electric field) is created, and an electric field in the lateral direction can be applied also to the liquid crystal molecules right above the pixel electrodes, whereby the liquid crystal molecules can be sufficiently driven. Therefore, a higher transmittance can be achieved at wide viewing angles than in the in-plane switching mode.
Further, in the liquid crystal display of the FFS mode, the liquid crystals are driven by the fringe electric field created between liquid crystal control slit electrodes and the pixel electrode disposed below the liquid crystal control slit electrodes with an inter-layer insulating film therebetween. In this configuration, a pixel aperture ratio can be prevented from being decreased when the pixel electrodes and the liquid crystal control slit electrodes are formed of an oxide-based transparent conductive film such as ITO (Indium Tin Oxide) containing indium oxide and tin oxide, or InZnO containing indium oxide and zinc oxide.
Further, because holding capacitors are formed by the pixel electrodes and the liquid crystal control slit electrodes, it is not always necessary to separately form a pattern for the holding capacitors within the pixels, differently from the liquid crystal display of the TN mode. This arrangement can achieve a high pixel aperture ratio.
Further, for switching devices of a TFT substrate for a liquid crystal display, amorphous silicon (a-Si) is conventionally used as a semiconductor material for a channel layer. Major reasons for that include the facts that a film having high uniform properties can be formed even on a large region substrate because amorphous silicon is amorphous and that, because a film can be formed at relatively low temperatures and the TFT substrate can be manufactured even on a low cost glass substrate whose thermal resistance is not good, the TFT substrate is highly suitable for liquid crystal displays for typical television sets.
However, in recent years, TFTs are being actively developed using an oxide semiconductor for a channel layer. The oxide semiconductor can stably provide an amorphous film having high uniformity when the composition is optimized, and has a higher mobility than conventional a-Si; therefore, the oxide semiconductor has an advantage that small-sized high-performance TFTs can be achieved. Therefore, application of such an oxide semiconductor film to a TFT substrate of the above FFS mode provides an advantage that it is possible to achieve an FFS mode TFT substrate having a higher pixel aperture ratio.
The TFT in which a-Si is used for a channel layer has a back channel etching (BCE) structure in which a channel region of the channel layer is exposed to wet etching when a source electrode and a drain electrode are formed. However, if an oxide semiconductor is applied to the BCE structure TFT, the oxide semiconductor is also etched by the wet etching of the source electrode and the drain electrode, whereby a channel cannot be formed.
To solve this problem, in Patent Document 1, an channel protective Si film is formed on an oxide semiconductor channel. With this configuration, the oxide semiconductor is not exposed to the wet etching, of the source electrode and the drain electrode, after forming the channel protective film, and it is thus possible to form the oxide semiconductor channel. Therefore, a TFT substrate can be configured by using TFTs whose channels are made of an oxide semiconductor.